The present invention relates to data storage and data communications systems, and more particularly to rotating disk data storage devices. Even more particularly, the present invention relates to error correction systems used within the controllers of disk data storage devices and data communication systems.
Since the advent of digital computing techniques, attention has focused on methods for reducing errors in data. Such errors may be attributable to transient conditions in a computing apparatus or transmission channel, called "soft" errors; or they may be recurrent errors, such as those resulting from defects in data storage media, etc., called "hard" errors. When the data storage media has a small defect within a track of data, modern controllers can skip over the defective area.
Rotating disk data storage devices are typically organized in a series of tracks, with the read/write head of such devices being movable between tracks. Within each track data is stored in sectors, and typically each sector holds a consistent number of bytes. While the media is being formatted, it is also analyzed to determine where defect areas are located. Each sector has an ID field, and the integrity of the sector is coded within the ID field of each sector. When the data is being written or read, these defective sectors can be skipped.
Prior art circuits can detect that a defect area is approaching, and write, prior to the defect, the accumulated Error Correction Code (ECC) Redundancy data field for the partial sector data field written up to the point of the defect. After the defect, these prior art circuits accumulate a second field of redundancy data which is then written after the remaining data in the sector. Thus when a defect exists in a data sector, data written in that sector is written in two separate data packets, each having its own error correction code redundancy field. Reading this information requires extra firmware in the controller to handle multiple redundancy fields within a sector. Also, multiple redundancy fields require extra disk space for storage and thus reduce the amount of usable data that can be written on a track.
This problem is exacerbated if multiple defects occur within a sector, so most prior art techniques simply mark any sector having a defect as unusable. They then substitute another sector, located elsewhere on the disk, for the defective sector. This causes a significant reduction in performance each time a data transfer with the defective sector is attempted, since the drive must find the substitute sector in order to accomplish the transfer.
Increasingly, data is being written using zone recording, where the data is written with a constant linear density on the media. That is, data written on the outside tracks of the media must be written at a higher frequency in order to maintain the same recording density on each track, since the outside tracks pass under the read/write heads at a higher rate of speed. This increases the amount of data that can be written on the outside tracks. Servo positioning information, however, is typically written in fixed bands radiating out from the center of the disk, so there are the same number of physical sector areas on each track regardless of the density being written. Thus, to write more data on the outside tracks requires that more data be written in the data field of each physical sector area, and often requires that some data fields contain partial data packets. That is, an outside physical sector area will contain more data than a sector area near the center of the disk, but not necessarily an integer multiple of the amount of data contained in an inside sector, therefore sector data will span across an intervening servo field. In these cases, the data is broken into multiple packets, and in prior art circuits, each packet must have a redundancy data field stored with it.
It is thus apparent that there is a need in the art for an improved method or apparatus which allows data to skip a defective area of the media without accumulating and storing multiple redundancy fields. There is a further need in the art for an apparatus and method that will allow data to span a servo field between adjacent physical sector areas without requiring that multiple redundancy data fields be accumulated and stored.